Modern technologies may include many processes that need to be controlled and automated. Using the example of CTP systems, these processes may comprise, for example: plate loading, plate unloading, conveying, centering and punching, drum balancing, etc. Electronic controllers are used to control these tasks.
A number of electronic controllers in use today are based on microprocessors that use software for defining the operation of the system. However, because of the different sensor types with different signal conditioning hardware or different loads with different driver types, use of the same controller board in different machines is often not feasible through changes in software only. Therefore, in present designs, the desired control functions are achieved by modifying the design and providing new circuitry for each model. This implies that a control system made for one type of sensor or load will not work with another one without requiring significant changes to the hardware.
An additional constraint of existing electronic controllers, which are programmed to perform desired control actions, is that microprocessors are limited in terms of interfaces. This limitation is caused by the fact that the architecture of microprocessors is computer-oriented rather than control-oriented. The actual environment of a control application typically involves the processing of a significant number of signals and devices (e.g. sensors, switches, motors, etc.), some of which are analog in nature. Consequently, a significant amount of circuitry is required in addition to the microprocessor, such as buffers, decoders, drivers, latches, multiplexers, analog-to-digital and digital-to analog converters, signal conditioning circuitry, etc.
FIG. 1 shows an example of typical hardware solutions needed to provide acquisition of different signal types.
The solution for motor encoder 10 logic signals includes filtering by filter 70 and buffering by logic input buffer 130 (low pass filter 70 should have a relatively high cut-off frequency to pass the encoder signals).
To receive the signals of slotted optical switches 20 with open collector output, a pull-up resistor 80 and a buffer 140 are needed. Also, a low pass filter (not shown) can be inserted.
For interlock door switches 30, usually used in standard 12V or 24V voltage supply safety loop, a voltage divider 90 is needed to reduce the input voltage signal to the level of logic supply (generally 5 or 3.3V), low pass filter 150 is needed to reduce bouncing and external noises (the cut off frequency of such a filter is different from the cut off frequency of filter 70 and buffer 160 is needed for fitting the signal to the configurable logic of digital unit 200.
One of the options for transferring analog signals, for example two pressure sensor signals 40 and 50, includes multiplexer 100, connected to ADC (Analog to Digital Converter) 170.
Some applications utilize reed switches with built-in LED indicator 60 e.g. NORGREN magnetically operated switch type QM/34. In this case, the solution may be based on a filter 110 and voltage comparator 190 using reference 180 as a threshold.
If the machine has a modular architecture, for example a manual machine, which by providing additional controlled subsystems can be upgraded to semiautomatic or fully automatic, the automation process may be realized either by a single multi I/O controller intended for all possible machine configurations or by three different smaller controllers, each dedicated to the manual, semiautomatic or fully automated configuration of the machine, respectively.
The drawback of the single multi I/O controller is in its inefficient utilization, especially in manual machines, since the controller hardware intended for semiautomatic and automatic modules is not used.
The drawback of three different dedicated controllers is in their higher service expenses and relatively high cost.
Let us assume that the target is to design a controller for a manual CTP machine (minimum automation level) comprising plate position detection, plate loading and plate unloading subsystems. The required subsystems include one motor encoder, ten slotted optical sensors with open collector output, and six door interlock switches.
The proper hardware solution for signals acquisition according to FIG. 1 will be the following:
For the motor encoder (signals ENCODER A and ENCODER B)—two filters 70 and two logic input buffers 130 are needed.
For ten slotted optical sensors with open collector output—ten pull up resistors 80 and ten logic input buffers 140 are suitable. Also, ten low pass filters (not shown) could be inserted.
For six door interlock switches—six voltage dividers 90, six filters 150 and six logic input buffers 160 are needed.
In order to be upgraded to semiautomatic, the CTP machine will need additional hardware, to provide signal acquisition and control for the new plate centering and plate punching automation subsystems containing, for example, three DC motors with encoders, eight slotted switches, two analog pressure sensors and two proximity sensors.
In this case, the controller should support six (3×2) encoder inputs instead of the two for the manual machine, eight slotted switches inputs instead of 10 and should have circuitry for supporting two analog and two plate short sensors, which were not used in the manual machine.
FIG. 2 shows an example of typical hardware solutions needed to provide control of different load types.
A possible solution for driving stepper motor 55 may be implemented by integrated stepper motor controller 120 (e.g. L297 stepper motor controller of SGS-Thomson microelectronics), connected to the outputs of the configurable digital unit 200 and driver 145 (e.g. L298—dual full bridge driver of SGS-Thomson Microelectronics), connected between stepper motor controller 120 and stepper motor 55. Such stepper motor control dedicated hardware can not be utilized for DC motor or valve control, which may be needed for different control environments of another model, for example for semiautomatic or fully automatic machines.
Similarly, the DC motor driver 115 (e.g. DMOS full-bridge PWM motor driver 3948 of ALLEGRO MicroSystems, Inc) connected between the configurable digital unit 200 and the DC motor 50, can not be used for separate valves or relay control, and the low drivers 155 (e.g. 6810 Latched Source Driver of ALLEGRO MicroSystems, Inc), controlled by configurable digital unit 200 and driving the 20 mA valves 60, can not be used for high current control solenoid driving application because of low current capability.
As can be seen, the controller hardware chosen for the manual machine cannot be used for controlling the subsystems of the semiautomatic machine, because of the differences in hardware solutions for acquisition of different sensor types and for controlling the different types of loads.
A similar situation occurs when upgrading from semiautomatic to fully automatic, or when trying to use the controller for a different family of machines.
Published U.S. patent applications Ser. No. US2001/0015918 and US2001/0039190 attempt to improve the above-mentioned shortcomings by providing a configurable electronic controller, comprising control circuitry for providing control functions, input interface, output interface, user interface, power interface and a non-volatile memory unit connected to the control circuitry and to all the above interface units, in order to configure them.
However, there are some barriers to the use of this solution in more complex machines, such as computer to plate (CTP):
1. The inputs of microcontrollers are not yet universal. This means that an analog sensor can not be connected to the digital input of the controller;
2. The digital inputs of a microcontroller can not provide the input hysteresis adjustment and input threshold control which are required for accepting input from sensors with different low and high logic levels, (for example, the low logic level of NORGREN magnetically operated switch type QM/34 is in the range of 2V, because of a built-in LED indicator. At the same time, the minimum input HIGH voltage of digital input buffer MOTOROLA SN74LS240 is 2V. Such difference in voltage levels can cause a faulty signal acquisition of QM34);
3. The provided solution involves hardware redundancy on the controller board; the controller architecture needs to include all possible functional blocks, configured by NV memory, to cover the different sensors' acquisition and, depending on the configuration, some of the functional blocks will not be used;
4. The provided solution provides a relatively low range of control. For example, the multiplexed signal acquisition of the controller leads to decrease in the controller response time. This disadvantage may become critical when dealing with e.g. CTP device controller, possibly handling over 100 sensors;
5. Using one fixed type of switch connected to one side of the load limits the load connection options (if the switch is high-side type, then the other side of the load should be connected to the common (ground) and if the switch is low-side type, then the other side of the load should be connected to the supplied voltage source. Generally, both load connections (to common and to voltage source) are used, but the mentioned Patent Applications can only support one kind of load connections;
6. No possibility to change current direction of loads needed, for example, for reversing DC motors.
Thus, there is a need for a universal, flexible controller architecture to enable improved upgradeability and serviceability and facilitate adaptation of an existing controller to a new machine.